The present invention relates to an image forming apparatus.
A conventional image forming apparatus such as a printer, a copier, a facsimile, a multi-function product, and the likes is provided with image forming units in colors such as black, yellow, magenta, and cyan. Accordingly, it is possible to switch between a color printing operation and a printing operation in one specific color, for example, a monochrome printing operation.
When the conventional image forming apparatus performs the color printing operation, a sheet contacts with a photosensitive drum as an image supporting member disposed in each of the image forming units. When the conventional image forming apparatus performs the monochrome printing operation, a sheet contacts with a photosensitive drum as an image supporting member disposed in the image forming unit of black. At this time, it is arranged such that the image forming units of the other colors move upwardly with a cam and the likes, so that the sheet does not contact with photosensitive drums of the image forming units of the other colors. Accordingly, it is possible to prevent the image forming units of the other colors from operating, thereby reducing wear or deterioration of the photosensitive drums thereof (refer to Patent Reference).
Patent Reference: Japan Patent Publication No. 2006-78544
In the conventional image forming apparatus described above, when the color printing operation is switched to the monochrome printing operation, it is necessary to move the image forming units of the other colors from a medium transportation path while the image forming unit of one color remains on the medium transportation path. When the monochrome printing operation is switched to the color printing operation, it is necessary to place the image forming units of the other colors on the medium transportation path, thereby taking a relatively long period of time.
When it is necessary to perform both the monochrome printing operation and the color printing operation, it may be arranged such that the image forming units of the other colors do not move from the medium transportation path, i.e., the state in the color printing operation.
In each of the image forming units, a developing blade as a developer regulating member is disposed to abut against a developing roller. The developing blade charges toner as developer through friction therebetween, so that toner is attached to a static latent image formed on the photosensitive drum, thereby forming a toner image as a developer image.
Occasionally, when the developing blade charges toner, a small amount of toner tends to be charged with an opposite polarity. In this case, a small amount of toner charged with an opposite polarity, or opposite charged toner, does not spontaneously adhere to a sheet due to a potential balance. However, when the opposite charged toner physically contacts with the sheet, the opposite charged toner may adhere to the sheet through van der Waals attraction.
When a small amount of toner adheres to the sheet in the printing operation, toner may cover a background of the sheet, i.e., a phenomenon called a fog, thereby deteriorating image quality.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus. In the image forming apparatus of the present invention, it is possible to perform a printing operation without causing a fog, thereby improving image quality.
Further objects of the invention will be apparent from the following description of the invention.